Magnetic recording medium and its manufacturing method and magnetic recording system using such a magnetic recording medium

ABSTRACT

Disclosed herein are a magnetic recording medium having a high coercive force and being capable of high-density writing/reading, a magnetic recording apparatus equipped with said magnetic recording medium, and a process for producing said magnetic recording medium.  
     The magnetic recording medium is composed of a substrate, a soft magnetic layer, a non-magnetic intermediate layer, a magnetic layer, a protective layer, and a lubricating layer. The magnetic layer is characterized by the product of the stacking fault density and the dispersion of particle diameters which is no larger than 0.02. The stacking fault density should preferably be no larger than 0.05, and the dispersion of particle diameters should preferably be no larger than 0.4.  
     The magnetic recording medium has a coercive force larger than 4000 Oe, is highly stable to thermal decay, and has a recording density in excess of 50 Gbit/in 2 .

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a perpendicular magneticrecording medium, a process of production thereof, and a magneticstorage equipped therewith. The perpendicular magnetic recording mediumhas a magnetic layer composed of columnar magnetic crystal grains whoseprincipal component is cobalt, and it is characterized by reducedthermal decay. The magnetic storage has a recording density in excess of50 Gbit/in².

[0003] 2. Description of the Related Arts

[0004] There is an increasing demand for higher recording density inmagnetic storage from the standpoint of increasing the storage capacity,miniaturizing the apparatus, and reducing the number of parts. Theexisting magnetic recording medium is based on longitudinal magneticrecording. It records information by means of mutually opposed domains(recording bits) which are magnetized in the direction parallel to thesurface of the substrate. For a longitudinal magnetic recording mediumto be capable of high-density recording, it should have a low noiselevel. One effective way of noise reduction is finely reducing in sizeof crystal grains and even out particle diameters (or reduce thedispersion of particle diameters). This is exemplified by the invention(disclosed in Japanese Patent Laid-open No. 269548/1998) relating to alongitudinal recording medium which specifies for noise reduction theoptimum particle diameter and the optimum dispersion of particlediameters.

[0005] Increasing the recording density in longitudinal magneticrecording will have a limit because of the necessity for more finelyreduced crystal grains than before. However, extremely small crystalgrains encounter problems with thermal decay. In other words, theirmagnetization for recording is decayed by even such small thermal energyas generated at room temperature. In order to address the problem withthermal decay, there has been proposed the perpendicular magneticrecording system, which is attracting great attention. It is essentiallysuitable for high-density recording by virtue of its property that thethermal stability of magnetization improves as the recording densityincreases.

[0006] The recording medium for perpendicular magnetization under widestudy is one which has a magnetic layer composed of practically columnarcrystal grains, with their (00.1) plane oriented nearly parallel to thesurface of the substrate for their magnetic anisotropy in the directionperpendicular to the substrate. The most widely studied recording mediumwith a CoCr alloy magnetic film has a coercive force of about 3000 Oe(equivalent to approximately 79.7 A/m in SI unit). In order to put topractical use the magnetic recording medium composed mainly of cobalt,the present inventors carried out extensive studies, which led to animportant finding that a magnetic film of cobalt alloy with the c-axis(or the (00.1) direction) oriented perpendicular to the surface of thesubstrate has a stacking fault density which is two to three timeshigher than that of cobalt alloy magnetic film with longitudinalorientation.

[0007] The perpendicular magnetic recording also needs a magneticrecording medium with low noise and high thermal stability. No matterwhether it is of longitudinal recording type or perpendicular recordingtype, the magnetic layer with many stacking faults will be poor inmagnetic anisotropy, coercivity, and thermal stability. For high thermalstability of recording magnetization, it is necessary to reduce thestacking fault density in the magnetic film.

[0008] The major cause of stacking faults is ingression of a planecorresponding to the fcc-like structure into the hcp structure. It isbelieved that the stacking fault density will decrease if the magneticfilm is formed at a low temperature desirable for the hcp structure tobe stable. However, the present inventors' elucidation suggests that themagnetic film formed at low temperatures decreases in stacking faultdensity but does not increase in coercive force because crystal grainsconstituting the magnetic film have a broad distribution of particlediameters. Raising the film-forming temperature to reduce the dispersionof particle diameters increases the stacking fault density, withcoercivity remaining low.

[0009] The longitudinal magnetic recording medium is inherently littlesubject to stacking faults because crystal grains constituting themagnetic layer are epitaxially grown on the underlayer such that thec-axis of magnetic grain is longitudinally oriented. Epitaxial growthtakes place in the direction toward the most stable energy state. Thus,there is almost no possibility that epitaxial growth brings about anunstable energy state due to ingression of a crystal phase differentfrom that of the fcc structure.

[0010] It was found from the present inventors' investigation that thestacking fault density in the longitudinal magnetic recording mediumformed at about 250° C. is one half to one-third of that in theperpendicular magnetic recording medium. It was also found that thedispersion of particle diameters in the longitudinal magnetic recordingmedium is about 0.3 to 0.4, which is determined almost entirely by thedispersion of particle diameters in the underlying film of chromiumalloy and which does not depend on the film-forming temperature.

[0011] The film-forming temperature affects the stacking fault densityand the dispersion of particle diameters as shown in FIGS. 1 and 2respectively. It is to be noted from FIG. 1 that the stacking faultdensity decreases as the film-forming temperature decreases. It is alsoto be noted from FIG. 2 that the dispersion of particle diametersincreases as the film-forming temperature decreases. This suggests thatit is difficult to have both of a low stacking fault density and a lowdispersion of particle diameters. Such an antinomic relation of thefilm-forming temperature with the stacking fault density and thedispersion of particle diameters has never been anticipated in thetechnology of longitudinal magnetic recording medium.

OBJECT AND SUMMARY OF THE INVENTION

[0012] It is an object of the present invention to provide aperpendicular magnetic recording medium which is made of conventionalCoCr alloy as a magnetic material and yet has good thermal stabilityowing to adequate control over the stacking fault density and thedispersion of particle diameters. Being made of a conventional magneticmaterial, the recording medium is economically advantageous.

[0013] It is another object of the present invention to provide a methodof adequately controlling the stacking fault density and the dispersionof particle diameters.

[0014] The objects of the present are achieved by controlling thestacking fault density (R) and the dispersion of particle diameters(ΔD/<D>) of the magnetic film, which is composed of magnetic crystalgrains whose principal component is cobalt, such that the product ofΔD/<D>×R is no larger than 0.02.

[0015] The stacking fault density relates with coercive force as shownin FIG. 3. Incidentally, dotted lines in FIG. 3 are contour lines forsome values of the dispersion of particle diameters which are determinedby the coercive force and the stacking fault density. It is noted fromFIG. 3 that the coercive force increases as the stacking fault densitydecreases and the coercive force slightly decreases as the stackingfault density decreases further (due to increase in the dispersion ofparticle diameters).

[0016]FIG. 4 shows the contour lines of coercive force which aredetermined by the stacking fault density and the dispersion of particlediameters. It is noted that the coercive force no smaller than 4000 Oeis obtained in the area (under the thick line) in which the product ofthe stacking fault density and the dispersion of particle diameters isno larger than 0.02. The coercive force of 4000 Oe is necessary tosuppress thermal decay. With a value of coercive force no smaller than4000 Oe, the magnetic recording medium has a value of Ku·V/k·T nosmaller than 60 which is necessary to ensure thermal stability forrecording magnetization. This value is a parameter to indicateresistance to thermal decay. (In Ku·V/k·T, Ku denotes a magneticanisotropy energy possessed by crystal grains, V denotes a volume ofcrystal grains, k denotes the Boltzmann constant, and T denotes anabsolute temperature.) According to the present invention, thisparameter Ku·V/k·T should have a value no smaller than 60; otherwise,the magnetic recording medium is not of practical use because itremarkably decreases in the amount of recorded magnetization. To achievethis object, the magnetic recording medium should have a coercive forceno smaller than 4000 Oe in view of the fact that the existingcobalt-based magnetic material has a value of about 0.38 T (tesla) forsaturation magnetization and the magnetic layer has a thickness of 18 nmand the magnetic crystal grains have an average particle diameter ofabout 12 nm (both attainable by the present technology).

[0017] One way to decrease both the stacking fault density and thedispersion of particle diameters is to form the magnetic film at atemperature no lower than about 250° C. and then anneal the resultingmagnetic film. Film forming at a high temperature increases the stackingfault density, but annealing decreases the stacking fault density.

[0018] The following is a probable reason why the stacking fault densityis decreased by annealing. The magnetic film, which is formed usually bysputtering, is subject to stacking faults because sputtering, which is anon-equilibrium process, does not permit atoms constituting the magneticfilm to diffuse completely into the film. In other words, sputteringfails to arrange atoms at energy-stable positions (or hcp lattices) butresults in stacking faults. Annealing after film formation moves atomsto the hcp lattice positions.

[0019] The present invention is also directed to a magneticrecording/reading unit which is constructed of the above-mentionedmagnetic recording medium, a drive mechanism to convey the recordingmedium, and a magnetic head for record writing/reading, which is acomponent capable of producing a high magneto-resistive effect. Themagnetic recording/reading unit has a recording density in excess of 50Gbit/in². The magnetic head should preferably be one which utilizes thegiant magneto-resistive effect, spin-valve effect, or tunnelingmagneto-resistive effect.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1 is a diagram showing the relation between the stackingfault density and the film-forming temperature in the conventionaltechnology.

[0021]FIG. 2 is a diagram showing the relation between the dispersion ofparticle diameters and the film-forming temperature in the conventionaltechnology.

[0022]FIG. 3 is a diagram showing the relation among the stacking faultdensity, the dispersion of particle diameters, and the coercive force inthe conventional technology.

[0023]FIG. 4 is a diagram showing the relation among the stacking faultdensity, the dispersion of particle diameters, and the coercive force inthe conventional technology.

[0024]FIG. 5 is a schematic diagram illustrating how to calculate thestacking fault density.

[0025]FIG. 6 is a schematic diagram illustrating how to calculate thedispersion of particle diameters.

[0026]FIG. 7 is a diagram showing the relation between the coerciveforce and the variation of angles of the c-axis in the conventionaltechnology.

[0027]FIG. 8 is a schematic diagram showing the cross-section of themagnetic recording medium pertaining to the present invention.

[0028]FIG. 9 is a diagram showing the relation among the stacking faultdensity, the dispersion of particle diameters, and the coercive force inExample 1 of the present invention.

[0029]FIG. 10 is a diagram showing the relation among the stacking faultdensity, the dispersion of particle diameters, and the coercive force inExample 2 of the present invention.

[0030]FIG. 11(a) is a plan view of the magnetic recording unitpertaining to the example of the present invention. FIG. 11(b) is asectional view (taken along line A-A′) of the magnetic recording unitpertaining to the example of the present invention.

[0031]FIG. 12 is a schematic sectional view showing the magnetic head ofwrite-read separate type.

[0032]FIG. 13(a) is a histogram showing the distribution of stackingfaults in the thickness direction of the magnetic film formed at 214° C.FIG. 13(b) is a histogram showing the distribution of stacking faults inthe thickness direction of the magnetic film formed at 330° C.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0033] The embodiments of the present invention are described below withreference to the accompanying drawings.

[0034] To start with, let us define the stacking fault density and thedispersion of particle diameters as follows.

[0035] In the examples of the present invention, the stacking faultdensity is obtained from the image of the sectional structure of asample of the magnetic film. The sectional structure is perpendicular tothe surface of the substrate, and the image is taken by a transmissionelectron microscope. Microscopic observation is carried out to give animage of crystal structure, with the object aperture and the defocusamount adequately controlled. The image of sectional structure thusobtained is scrutinized to select the particle on which the (11.0) planeis apparently visible. Layers having the (00.2) plane in the selectedparticle are sequentially examined, starting from the one adjacent tothe surface of the substrate. The number of planes not assuming the hcpstructure is counted. The plane not assuming the hcp structure isdefined as shown in FIG. 5. The hcp structure consists of identicallayers stacked one over the other in the c-axis direction. If eachequivalent layer is designated as A and B, the layer structure may bedelineated as A,B,A,B,A,B, . . . However, the layer structure withstacking faults may be delineated as A,B,A,C,A,C, . . . , for example,where C denotes a third equivalent layer. The middle portion indicatedby B,A,C constitutes the stacking fault, and it is the plane notassuming the hcp structure. This portion is counted as one plane withstacking fault. The latter half portion indicated by C,A,C assumes thehcp structure, and hence it does not constitute the stacking fault. Thenumber of planes with stacking faults divided by the total number of the(00.2) plane in the selected particle is then defined as the stackingfault density on the selected particle. The same measurement as above isrepeated for a plurality of particles. The average of the results is thestacking fault density of the magnetic film of a given sample.

[0036] The dispersion of particle diameters is also calculated from animage taken by a transmission electron microscope as follows. First, aplan view image of the magnetic film is observed with a transmissionelectron microscope, and crystal lattice image observed from thedirection perpendicular to the surface of the substrate arephotographed. The map of the grain boundary produced from the latticeimage thus obtained is schematically shown in FIG. 6. The map of thegrain boundary is scanned so as to count the number of pixels presentwithin the boundary of one crystal grain. The number of pixelsmultiplied by the scale for conversion into area gives the area of onecrystal grain. The diameter of the circle having the same area as theobtained grain area is regarded as the particle diameter of the crystalgrain. This calculation is performed on about 300 crystal grains. Theaverage of the particle diameters thus obtained is designated as averageparticle diameter <D>. Also, the standard deviation of the averageparticle diameter is calculated. It is designated as ΔD. The standarddeviation is divided by the average particle diameter to give thedispersion of particle diameters, which is designated as ΔD/<D>.

EXAMPLE 1

[0037] This example demonstrates a perpendicular magnetic recordingmedium, whose schematic sectional view is shown in FIG. 8. Theperpendicular magnetic recording medium is composed of a substrate 6, asoft magnetic layer 5, a non-magnetic intermediate layer 4, a cobaltalloy magnetic layer 3, a protective layer 2, and a lubricating layer 1,which are arranged one over another. Incidentally, there is interposed aNiTaZr film between the substrate 6 and the soft magnetic layer 5 forbetter adhesion between them. The substrate 6 may be formed fromchemically strengthened glass, crystallized glass, amorphous carbon,Al—Mg alloy (with NiP plating), or the like.

[0038] Each layer was formed on the crystallized glass substrate 6 bymagnetron sputtering. First, on the substrate 6 was formed anintermediate layer (30 nm thick) from NiTa(37.5)Zr(10), which iscomposed of 52.7 at % Ni, 37.5 at % Ta, and 10 at % Zr. (This notationis used hereinafter.)

[0039] Then, the soft magnetic layer 5 (400 nm thick) was formed fromFeTa(10)C(16). Other soft magnetic materials include FeTaC, FeTaN, andCoTaZr. The soft magnetic layer may be formed with its direction ofmagnetization fixed, or it may also be formed in combination with one ormore layers which control crystalline characteristics.

[0040] The surface of the soft magnetic layer 5 was heated to 330° C. byusing an infrared lamp so that the substrate acquired a temperature highenough for film forming. The non-magnetic intermediate layer 4, themagnetic layer 3, and the protective film 2 were formed sequentially. Ifthe temperature of the substrate is not higher than 250° C., thedispersion of particle diameters would be larger 0.4 as shown in FIG. 2.Such a condition should be avoided.

[0041] The non-magnetic intermediate layer 4 is a laminate consisting ofa 4-nm thick film of NiTa(37.5)Zr(10) and a 1-nm thick film of CoCr(40).The former film permits the latter's c-axis to orient in theperpendicular direction and also makes the latter's crystal grains fine.The effect is that the magnetic layer formed on the CoCr film iscomposed of fine crystal grains.

[0042] Incidentally, the above-mentioned non-magnetic intermediate layer4 (in laminate form) may be replaced by a single non-magnetic film ofNiTa, NiTaZr, CoCr, CoCrB, CoCrB, CoB, CoRu, TiCr, or the like.

[0043] On the CoCr film was formed by sputtering the magnetic layer 3which is a 18-nm thick film of CoCr(19)Pt(14). Sputtering was carriedout at an argon pressure of 0.5 Pa and a film-forming rate of 7.8 nm.The maximum degree of vacuum that was attained by the sputtering chamberwas 5 μPa. The protective layer 2 has a thickness of 5 nm. The samplefor performance evaluation was given the lubricating layer 1.

[0044] The raw material for the magnetic layer 3 may include, inaddition to the above-mentioned CoCr(19)Pt(14), CoCrPt alloys and CoCrPtalloys incorporated with one or more elements such as Ta, B, Nb, and Cu.Their examples are CoCr(19)Pt(14), CoCr(22)Pt(14), andCoCr(17)Pt(14)B(4). Chromium in the magnetic film segregates more ingrain boundaries than in grain interior, thereby lowering magneticcoupling between grains. Platinum enhances the magnetic anisotropy ofcrystal grains. The additional elements reduce the crystal particlediameter and lower magnetic coupling between adjacent particles.

[0045] The thus formed magnetic film was heated at 370° C. for 12seconds by means of an infrared lamp.

[0046] The sample was allowed to stand for 120 seconds in the evacuatedchamber. Then, on the magnetic film was formed the protective layer 2from carbon or a carbonaceous material containing H or N. The protectivelayer 2 is usually 2-5 nm thick. The protective layer 2 was furthercoated with the lubricating layer 1 (2-10 nm thick) of perfluoroalkylpolyether. Thus, there was obtained a highly reliable magnetic recordingmedium.

[0047] For comparison of magnetic characteristics, comparative samplesof magnetic recording medium were prepared in the same way as mentionedabove, except that annealing by an infrared lamp was not performed orthe stacking fault density was varied by adjusting the film-formingtemperature.

[0048]FIG. 9 shows a comparison between the sample in this example andthe comparative sample. It is to be noted that the sample in thisexample has a stacking fault density of 0.048 and a dispersion ofparticle diameters of 0.40, both measured according to theabove-mentioned method. These values are remarkably lower than those ofthe comparative examples. The sample in this example was tested formagnetostatic properties by using a vibrating sample magnetometer. Itwas found to have a coercive force of 4300 Oe and a squareness of 0.97,which are highly desirable values. This result suggests that the samplehas a coercive force higher than 4000 Oe owing to its stacking faultdensity lower than 0.05 and its dispersion of particle diameters smallerthan 0.4.

[0049] The magnetic recording medium obtained in this example wasmounted on a magnetic recording apparatus for evaluation of read/writecharacteristics. As shown in FIG. 11, this apparatus consists of amagnetic recoding medium 111 and a drive 112 therefor, a magnetic head113 and a drive 114 therefor, and a means 115 to process write/readsignals to and from the magnetic head. As shown in FIG. 12, the magnetichead 113 consists of a main magnetic pole 101, a recording coil 102, anupper shield 103 (which functions also as an auxiliary pole), a giantmagneto-resistive element 104, and a lower shield 105. The magnetic head113 is of write/read separate type, and it is formed on the magnetichead slider. The write unit, which is the single-pole type head, has atrack width of 170 nm. The read unit has an effective track width of 124nm. The shield space is 60 nm.

[0050] The write/read test was carried out under the followingconditions.

[0051] Linear recording density: 769 kFCI (kilo flux change per inch)

[0052] Track pitch: 195 nm

[0053] Distance between the magnetic head and the magnetic recordingmedium: 15 nm

[0054] There was obtained an S/N ratio of 22.0 dB. This value is highenough for the magnetic recording medium to have a recording density inexcess of 50 Gbit/in².

[0055] Then, the sample was examined for thermal stability which iscrucial for high-density recording. This object was achieved bymeasuring the rate of decrease of the read output with time. Thisparameter is defined as a ratio of the difference between the readoutput measured immediately after recording and the read output measuredafter standing for a certain period to the read output measuredimmediately after recording. The rate of decrease was 2% when the readoutput was measured 100 hours after recording with a linear recordingdensity of 100 kFCI. This suggests that the sample will retain recordeddata for a long period of time. The sample was also tested for thedependence of residual coercivity on the duration of magnetic fieldapplication. This test gave a value of Ku·V/k·T no smaller than 60. Thisresult suggests that the sample has good thermal stability.

EXAMPLE 2

[0056] The same procedure as in Example 1 was repeated except thatannealing was carried out at a lower temperature and for a longer time.The resulting sample was tested for magnetic characteristics. There isan instance where annealing is performed on a number of magneticrecording media. In practice, however, it is usually impossible to holdall of them in a sputtering vacuum chamber with a limited capacity. So,it is necessary to remove them out of a vacuum chamber. This is thereason why the magnetic recording medium needs a protective film forprotection from oxidation. The protective film should preferably be madeof a carbonaceous material. To prevent carbon in the protective filmfrom diffusing into the magnetic film, it is necessary not to heat themagnetic medium above 250° C.

[0057] The sample of the magnetic recording medium in this example issomewhat similar in structure to that in Example 1. It is composed ofthe following layers.

[0058] Intermediate layer: a laminate composed of a 2-nm thick film ofNiTa(37.5)Zr(10) and a 3-nm thick film of CoCr(40).

[0059] Magnetic film (20-nm thick) of CoCr(17)Pt(14)B(4).

[0060] Protective film (5-nm thick) of carbon.

[0061] After film formation, the sample was allowed to cool for 8 hoursin a constant temperature oven at 220° C.

[0062] A comparative sample of magnetic recording medium was prepared inthe same way as in Example 1, except that annealing was not performed.

[0063]FIG. 10 shows a comparison between the sample in this example andthe comparative sample. It is to be noted that the sample in thisexample has a stacking fault density of 0.050 and a dispersion ofparticle diameters of 0.39, both measured according to theabove-mentioned method. These values are remarkably lower than those ofthe comparative example. The sample in this example was tested formagnetostatic properties by using a vibrating sample magnetometer. Itwas found to have a coercive force of 4000 Oe and a squareness of 0.90,which are highly desirable values. Incidentally, the sample in thisexample was examined by a transmission electron microscope before itsannealing (for 8 hours in a constant temperature oven). It was found tohave a stacking fault density of 0.15 and a dispersion of particlediameters of 0.39. This result indicates that annealing for an extendedtime at a comparatively low temperature greatly reduces the stackingfault density without changing the dispersion of particle diameters. Thesample in this example has a stacking fault density lower than 0.05 anda dispersion of particle diameters lower than 0.4, and hence it has acoercive force higher than 4000 Oe.

[0064] The magnetic recording medium obtained in this example wasmounted on a magnetic recording apparatus (shown in FIG. 11) forevaluation of read/write characteristics. The write unit, which is thesingle-pole type head, has a track width of 170 nm. The read unit has aneffective track width of 124 nm and a shield space of 60 nm. The readunit employs a giant magneto-resistive element. The write/read test wascarried out under the following conditions.

[0065] Linear recording density: 769 kFCI

[0066] Track pitch: 195 nm

[0067] Distance between the magnetic head and the magnetic recordingmedium: 15 nm

[0068] There was obtained an S/N ratio of 20.3 dB. This value is highenough for the magnetic recording medium to have a recording density inexcess of 50 Gbit/in².

[0069] Then, the sample was examined for the rate of decrease of theread output with time. The rate of decrease was 2% when the read outputwas measured 100 hours after recording with a linear recording densityof 100 kFCI. This suggests that the sample will retain recorded data fora long period of time.

EXAMPLE 3

[0070] This example demonstrates the effect that is produced bycontrolling the dispersion of angles of c-axis. One effective way toreduce the dispersion of particle diameters is to increase thedispersion of angles of c-axis of the columnar crystal grainsconstituting the magnetic film. The dispersion of angles of c-axis isdefined as the full width at half maximum of the dispersion of theangles which the (00.1) plane makes with the surface of the substrate.It is considered that the crystal grains constituting the magnetic filmgrow from the nuclei which have been randomly generated duringsputtering. If those crystal grains which have grown from such nucleibecome independent crystal grains, then the dispersion of particlediameters should be about 0.28 according to simulation with Voronoifigure. However, practical crystal growth takes place such thatadjoining particles coalesce into a single particle. This is trueparticularly with crystal grains with a small dispersion of angles ofc-axis. Crystals with six symmetries (as viewed in the directionperpendicular to the film surface) tend to form coalesced crystal grainsbecause there is a high possibility that the crystal plane orientationof one crystal coincides with that of its adjacent crystal. The resultis that the dispersion of particle diameters does not decrease as shownin FIG. 3 even though the substrate temperature is raised. Incidentally,this is not the case with longitudinal magnetic recording media in whichthe c-axis is oriented in the longitudinal direction and hence there isa low possibility of adjoining grains coalescing. Thus the dispersion ofparticle diameters is about 0.3. By contrast, in the case of magneticrecording medium with c-axis oriented in the perpendicular direction,the orientation of the c-plane (as viewed in the direction parallel tothe film surface) hardly coincides with that of adjoining crystals (oradjoining grains hardly coalesce) if the crystal grains are grown on ahighly irregular surface. In other words, under such conditions theformation of large particles will be suppressed and the dispersion ofparticle diameters will approach 0.28.

[0071]FIG. 7 shows the relation between the coercive force and thedispersion of angles of c-axis in the conventional technology. It isnoted that if the dispersion of angles of c-axis is smaller than 6degrees, the coercive force is 3000 Oe at the highest.

[0072] The dispersion of angles of c-axis is measured in the followingmanner by using an X-ray diffractometer (θ-2θ method). First, thediffraction peak due to the (00.2) plane is detected by changing theincident angle (θ) of X-rays. With the incident angle of X-rays fixedfor the thus obtained diffraction peak, the specimen is tilted and theintensity of X-rays detected at different tilting angles is plotted.Thus, there is obtained a rocking curve showing the distribution of peakintensities. The full width at half maximum of the curve is regarded asthe dispersion of angles of c-axis.

[0073] In this example, the magnetic recording medium has a softmagnetic film of FeTa(10)C(6), a non-magnetic intermediate laminatelayer consisting of a 2-nm thick film of NiTa(37.5)Zr(10) and a 3-nmthick film of CoCr(40), a 18-nm thick magnetic film of CoCr(19)Pt(14),and a 5-nm thick protective film of carbon. The soft magnetic film ofFeTa(10)C(6) was heated at 250° C. by an infrared lamp after itsformation. The non-magnetic film of NiTa(37.5)Zr(10) was formed bymagnetron sputtering with argon at a pressure of 3.5 Pa. The argonpressure was kept rather high so as to roughen the surface of the filmof NiTa(37.5)Zr(10). The reason for this as follows. It has been knownfrom cross sectional TEM observation that crystal grains constitutingthe magnetic film epitaxially grow on the film of CoCr(40). However, itis considered that the dispersion of angles of c-axis in the magneticfilm depends on the film of CoCr(40). When formed by the conventionaltechnology, the film of CoCr(40) has a dispersion of angles of c-axissmaller than about 5 degrees as shown in FIG. 7, whereas when formed onthe film of NiTa(37.5)Zr(10) with its surface roughened, the film ofCoCr(40) has a dispersion of angles of c-axis larger than about 5degrees. As the dispersion of angles of c-axis increases, there is alower possibility of adjoining grains coalescing, which leads to areduced dispersion of particle diameters.

[0074] On examination of the rocking curve obtained from X-raydiffractometry, the magnetic recoding medium in this example was foundto have a dispersion of angles of c-axis of 8 degrees. It was also foundto have a coercive force of 4100 Oe and a squareness ratio of 0.90.

[0075] After its test for magnetostatic characteristics, the magneticlayer was observed under a transmission electron microscope to examinethe crystalline structure on its surface and cross-section. It was foundthat the stacking fault density is 0.070 and the dispersion of particlediameters is 0.28. A probable reason why the stacking fault density ishigher than 0.05 is that the magnetic layer was not annealed after ithad been formed. Yet, the stacking fault density is still higher thanabout 0.11 which is obtained from FIG. 1 showing how the stacking faultdensity depends on the film-forming temperature. A probable reason forthis is that the intermediate layer of CoCr(40) stabilizes thecrystalline structure in the early stage of crystal growth of themagnetic film, thereby suppressing the stacking faults. The magneticrecording medium in this example was found to have the stacking faultdensity and the dispersion of particle diameters such that their productis smaller than 0.02. It was also found to have a coercive force as highas 4000 Oe. Incidentally, a comparative sample having an intermediatelayer of CoCr(40) without annealing for reduction in the dispersion ofparticle diameters has a coercive force of 3500 Oe, but it does not havea sufficient coercive force in excess of 4000 Oe.

[0076] The magnetic recording medium obtained in this example wasmounted on a magnetic recording apparatus (shown in FIG. 11) forevaluation of read/write characteristics. The write unit, which is thesingle-pole type head, has a track width of 170 nm. The read unit has aneffective track width of 124 nm and a shield space of 60 nm. The readunit employs a giant magneto-resistive element. The write/read test wascarried out under the following conditions.

[0077] Linear recording density: 769 kFCI

[0078] Track pitch: 195 nm

[0079] Distance between the magnetic head and the magnetic recordingmedium: 15 nm

[0080] There was obtained an S/N ratio of 21.4 dB. This value is highenough for the magnetic recording medium to have a recording density inexcess of 50 Gbit/in².

[0081] Then, the sample was examined for the rate of decrease of theread output with time. The rate of decrease was 2% when the read outputwas measured 100 hours after recording with a linear recording densityof 100 kFCI. This suggests that the sample will retain recorded data fora long period of time.

EXAMPLE 4

[0082] This example demonstrates the effect which is produced byreducing the amount of platinum. The sample of the magnetic recordingmedium in this example has a soft magnetic film of FeTa(10)C(6), a 5-nmthick intermediate film of NiTa(37.5)Zr(10), a 6-nm thick film ofCoCr(17)Pt(8) (as a first magnetic layer), a 12-nm thick film ofCoCr(19)Pt(16) (as a second magnetic layer), and a protective carbonlayer. The total thickness of the magnetic layer is 18 nm. The softmagnetic film of FeTa(10)C(6) was heated at 250° C. by an infrared lampafter its formation.

[0083] The sample was tested for magnetostatic characteristics by usinga vibrating sample magnetometer. It was found to have a coercive forceof 4000 Oe and a squareness ratio of 1.0. After its test formagnetostatic characteristics, the magnetic layer was observed under atransmission electron microscope to examine the stacking fault densityand the dispersion of particle diameters. It was found that the stackingfault density is 0.050 and the dispersion of particle diameters is 0.40.A probable reason for reduction in the stacking fault density is adecrease in the amount of platinum in the first magnetic layer. Thiswill be explained below in more detail.

[0084] The distribution of stacking faults in cross-section variesdepending on the film-forming temperature (214° C. and 330° C.) as shownin FIG. 13 (with the ordinate representing the incremental thickness offilm and the abscissa representing the number of stacking faults). It isnoted that the stacking fault density increases in the early stage ofcrystal growth in the magnetic film regardless of the film-formingtemperature. A possible cause for this is a stress that occurs in theinterface between the magnetic film and the intermediate film.

[0085] In the meantime, Journal of Magnetism and Magnetic Materials,vol. 152 (1996) pp. 265-273, reports the magnetic anisotropy whichoccurs in a magnetic layer of CoPt alloy for longitudinal recordingmedium formed on the substrate. The reported data indicate that themagnetic anisotropy rapidly decreases as the amount of platinum in theCoCr alloy exceeds 12 at %. This suggests a steep increase of stackingfaults in the magnetic layer. Therefore, it is considered that the upperlimit of platinum to be added to the magnetic layer is 12 at %. Thus, inthe case where the magnetic film is composed of two layers, the lowerfilm should be formed such that the amount of platinum is less than 12at %. In this way it would be possible to sufficiently decrease thestacking fault density in the initial stage of crystal growth. The lowstacking fault density of the magnetic film in this example may beattributable to the laminate structure of two magnetic layers, the firstlayer containing a less amount of platinum for reduction of stackingfaults and the second layer being isolated from the stress at theinterface of the intermediate film.

[0086] The magnetic recording medium obtained in this example wasmounted on a magnetic recording apparatus (shown in FIG. 11) forevaluation of read/write characteristics. The write unit, which is thesingle-pole type head, has a track width of 170 nm. The read unit has aneffective track width of 124 nm and a shield space of 60 nm. The readunit employs a giant magneto-resistive element. The write/read test wascarried out under the following conditions.

[0087] Linear recording density: 769 kFCI

[0088] Track pitch: 195 nm

[0089] Distance between the magnetic head and the magnetic recordingmedium: 15 nm

[0090] There was obtained an S/N ratio of 20.5 dB. This value is highenough for the magnetic recording medium to have a recording density inexcess of 50 Gbit/in².

[0091] Then, the sample was examined for the rate of decrease of theread output with time. The rate of decrease was 2% when the read outputwas measured 100 hours after recording with a linear recording densityof 100 kFCI. This suggests that the sample will retain recorded data fora long period of time owing to its good thermal stability.

[0092] [Effect of the invention] The magnetic recording medium of thepresent invention has a high coercive force and good resistance tothermal decay. Moreover, it will realize a magnetic recording apparatushaving a recording density in excess of 50 Gbit/in when it is used incombination with a magnetic head having a high magneto-resistive effect.

What is claimed is:
 1. A perpendicular magnetic recording medium comprising: a substrate, a magnetic layer composed of cobalt-containing magnetic crystal grains with hexagonal closed-packed (hcp) structure, which is formed above said substrate, wherein; the value of ΔD/<D>×R is no greater than 0.02, where said <D> is an average grain size of said magnetic grains, said ΔD is a standard deviation of the distribution of the size of said magnetic crystal grains, and said R is the stacking fault density of said magnetic layer.
 2. A perpendicular magnetic recording medium according to claim 1, wherein, said value of ΔD/<D> is no greater than 0.4.
 3. A perpendicular magnetic recording medium according to claim 2, wherein, the value of R is no greater than 0.05.
 4. A perpendicular magnetic recording medium according to claim 1, further comprising: a non-magnetic underlying layer with hcp structure formed under said magnetic layer.
 5. A perpendicular magnetic recording medium according to claim 1, said magnetic layer further comprising: a first magnetic layer containing at least cobalt and platinum, a second magnetic layer formed on said first magnetic film, wherein, the content of platinum in said first magnetic layer is no greater than 12 at %.
 6. A perpendicular magnetic recording medium according to claim 1, wherein, a full width at half maximum of a distribution of angles of which the (00.1) plane makes with the surface of the substrate has no greater than 8 degrees.
 7. A perpendicular magnetic recording medium according to claim 1, wherein, a perpendicular coercive force is no less than 4000 Oe.
 8. A method for producing a perpendicular magnetic recording medium, comprising a step of forming a cobalt-containing magnetic film directly or with an intervention of an underlayer on a substrate, and a step of heating said magnetic film.
 9. A method for producing a perpendicular magnetic recording medium according to claim 8, further comprising: a step of forming a protective layer on said magnetic layer, and a step of heating the thus formed layers under atmospheric pressure after the formation of said protective layer, wherein said step of heating is carried out with the heating temperature no higher than 250° C.
 10. A method for producing a perpendicular magnetic recording medium according to claim 8, further comprising: a step of forming a non-magnetic film with hexagonal closed-packed (hcp) structure before the formation of said magnetic film.
 11. A magnetic recording apparatus equipped with the perpendicular magnetic recording medium defined in claim
 1. 